Body-coupled communication (BCC) allows exchange of information between devices that are located at or in close proximity of a human or animal body. BCC signals are conveyed over the body instead of through the air. The body (and the space few centimeters around it) is utilized as a communication channel, thus allowing for touch-based interaction and data exchange. A detailed description of the basic underlying communication principle is given by Thomas Guthrie Zimmerman, “Personal Area Networks (PAN): Near-Field Intra-Body Communication”, MASTER OF SCIENCE IN MEDIA ARTS AND SCIENCES at the Massachusetts Institute of Technology, September 1995. In this thesis, the term “near-field intra-body communication” is used when describing a body-coupled or body-based communication.
A communication based on BCC signals is confined to an area close to the body. This is in contrast to radio frequency (RF) communications, where a much larger area is covered. Thus, when using BCC signals, a communication is only possible between devices situated on, connected to or placed close to the same body.
On the other hand, RF communication technologies using a high frequency like e.g. low power short range RF technologies operating in the 2.4 GHz industrial, scientific and medical (ISM) band such as IEEE 802.15.4 (“ZigBee”) technologies are well suited for off-body communication under line-of-sight conditions. However, they are not well suited to operate around the human body. They suffer from high body shadowing or attenuation leading to unreliable communication. This frequently happens in scenarios where two body sensors want to talk with each other or the human or animal body blocks the direct line-of-sight between an on-body sensor and an off-body device. For example, a node on the chest of a user cannot communicate to another node on the back of the user. As RF systems use crowded free frequency bands, they suffer from interference. This again decreases the reliability of the communication. Further, RF systems exhibit a very high power consumption leading to short system lifetime and expensive and cumbersome battery management.
Providing a reliable wireless connectivity is advantageous for healthcare applications and other applications relying on networked on-body sensors and/or actuators as well as off-body devices. One example of such applications is automatic fall detection (AFD) in houses of elderly persons by means of a body area network (BAN). Each elderly person wears sensors such as e.g. accelerometers to detect if the person falls. In that case an alarm is sent via a wireless infrastructure based on e.g. WiFi or Zigbee technology. In some scenarios, e.g. if the elderly person falls on one or more sensors, at least part of the sensors cannot communicate anymore with the wireless infrastructure. As a result, it can be impeded that any alarm is sent.
FIGS. 18 to 21 illustrate the body shadowing of a RF communication in an AFD application. FIG. 18 illustrates a normal case of RF communication. Back and front nodes BN, FN such as sensors attached to the back and front of an elderly person can communicate with an off-body destination d such as e.g. an access point (AP) in the form of a device mounted e.g. to a room ceiling. FIG. 19 illustrates a front case of RF communication. Only the front node FN is able to communicate with the destination d. FIG. 20 illustrates a back case of RF communication. Only the back node BN can communicate with the destination d. FIG. 21 illustrates a fall case of RF communication. Only the back node BN is able to communicate with the destination d.
Another example is patient monitoring in hospitals by using body-worn medical sensors such as e.g. electrocardiogram (ECG), pulse oximetry (SpO2) and blood pressure sensors. These sensors may wirelessly transmit their measurements via a short range radio to a nearby patient monitor (if the patient is lying in his bed) [“off-body communication”] or to a body-worn hub [“on-body communication”] that forwards the data via an infrastructure based on wireless local area network (WLAN) technology to a central nurse station (if the patient walks around the hospital). A medical-grade wireless connectivity is required for patient monitoring in hospitals.
FIGS. 22 and 23 illustrate an example of patient monitoring in hospitals. FIG. 22 illustrates an example of patient monitoring at the bedside. In this case, first and second sensors 1, 2 can directly communicate with a bedside monitor 3 that is connected to a patient information centre 4 via a network 5. A hub 0 is not used. FIG. 23 shows an example of patient monitoring while a patient is ambulating. In this case, the first and second sensors 1, 2 may communicate with a hub 0, which in turn can communicate with an AP 7 such as e.g. an off-body device mounted e.g. to a room ceiling and connected to the patient information centre 4 via the network 5.
A wireless link quality to an off-body device varies depending on the position of a short range radio on the body of the patient for transmitting sensor measurements. Further, the RF conditions change dynamically due to the movement of the patient.
FIGS. 24 and 25 illustrate the dynamically changing wireless link quality. FIG. 24 illustrates a scenario where the second sensor 2 has a better wireless link quality. The sensor 2 has a better wireless link quality to the bedside monitor 3 than the sensor 1, i.e. a link quality indication (LQI) of the sensor 2 is higher than a LQI of the sensor 1. That is, in the scenario of FIG. 24 LQI(1→3)<LQI(2→3) applies. FIG. 25 illustrates a scenario where the first sensor 1 has a better wireless link quality. The RF conditions are reversed, i.e. the sensor 1 has a better wireless link quality to the bedside monitor 3 (higher LQI) than the sensor 2. That is, in the scenario of FIG. 25 LQI(1→3)>LQI(2→3) applies.
On the other hand, BCC technologies based on e.g. capacitive coupling or bone conduction are well suited for on-body communication, but are unable to provide connectivity to off-body devices.
As a result, neither RF nor BCC alone can meet the connectivity requirements of body-worn sensors as demanded for healthcare applications such as e.g. patient monitoring in hospitals and other applications such as e.g. AFD. This fact has also been recognized by the IEEE 802.15.6 BAN working group, which aims for specifying three different wireless technologies for on-body, off-body and in-body communication, respectively.